Flavour is part of our primary sensory system that controls food intake1 so that we consume pleasant (i.e. nutritional food) and avoid unpleasant food (containing potential toxins). Flavour is a sensation formed from visual, taste, aroma and mouth feel inputs. However, food choice and the amount we consume seem to be driven more by three of the five basic tastes (salt, sweet and umami) and is less affected by the other flavour attributes. Foods containing these attributes tend to be the ones preferred by humans as well as most mammals; in that context umami serves as a marker for proteins and sweetness for carbohydrates.
Recently the receptors involved in the detection of these taste modalities have been identified and cloned2-4, thus making it possible to investigate activation of taste receptors in vitro. The receptors for sweet, umami and bitter belong to the class of G-protein coupled receptors (GPCRs), whereas saltiness and sourness are most likely detected by ion channels.
Sweetness is sensed by the heterogeneous receptor dimer T1R2/T1R3, whereas umami is primarily detected by the T1R1/T1R3 receptor2, although other receptors have also been implicated to be involved in umami as well5.
Various cellular systems can be used for measuring in vitro receptor activation with good correlation to the in vivo sensory perception, including heterologous expression of taste receptors in mammalian cell lines like HEK293 cells2, 6-9. The currently available functional in vitro screening systems usually make use of promiscuous G-proteins such as Gα15, Gα16 or chimeras of these G-proteins with various adaptations of the C-terminal domain; this will direct the signalling cascades of receptors of interest to PLC (phospholipase C) and release of intracellular calcium. Although this approach has been very successful for investigating pure compounds, it has proven to be more difficult for testing extracts or complex samples: due to the universal nature of the G-proteins they are not only able to couple to the recombinant receptors (over)expressed in the screening cell lines, but also to many receptors which are endogenously present at low levels. This can result in unspecific calcium signals induced by agonists present in natural mixtures activating these endogenous receptors. Moreover, extracts or complex test samples often also contain substances, which elevate intracellular calcium by other means than via GPCRs, and these signals will be indistinguishable from receptor-induced calcium release. The high unspecific background signal observed for most natural mixtures prevents direct screening of these samples without extensive fractionation procedures. It is to be noted that the use of such extracts or complex samples is quite common when evaluating food material for example.
Therefore there is still a need for an improved screening method for a potential modulator compound of a taste receptor, wherein this method does not have each of the drawbacks of existing methods.